White Dragon

The White Dragon is actually extremely powerful but he can be taken out quickly because of his low HP. However, he counters every physical attack by casting Slow on a party member (eventually, therefore, all of your characters will be going super slowly). Furthermore, if you cast Bahamut on him (which you should), he'll counter with Earthquake. Hence, have Rosa cast Float on the entire party in the first round, then have her defend and heal thereafter. Cecil, Kain and Edge should all attack physically, and Rydia should cast Bahamut. In a few rounds, the White Dragon will be no more. But of course, in relation to the other bosses in the Lunar Subteranne and Core, he's nothing.